1. Field of Invention
This invention pertains to the art of methods and apparatuses for measuring the quantity of a sample material utilizing photometric means, and more specifically to methods and apparatuses for obtaining and processing detector responses to both measuring and reference radiation.
2. Description of the Related Art
The measurement of material quantities is important in many industrial, medical, and domestic applications. The use of light interaction with sample materials provides a convenient, low cost, and reliable measuring technology known as photometry. The principles of photometry are well known in the art. The Beer-Lambert law of absorption is given below: ##EQU1## where I(.lambda.).sub.O is the radiation intensity at a specified wavelength measured when no absorbing material is present; I(.lambda.) is the measured radiation intensity at a specified wavelength after I(.lambda.).sub.O is passed through the sample material, .sigma.(.lambda.) is the probability of radiation absorption by the sample material at the wavelength .lambda.; n is the quantity of material to be measured; and l is the distance the radiation travels in the material medium. Under ideal conditions of a collimated monochromatic source with constant radiation intensity and a well defined path length of radiation, the Beer-Lambert law provides a convenient means of measuring the quantity of material in a sample.
The difficulties associated with a practical photometer include establishing a radiation source with a constant radiation flux at a detector at a specified radiation wavelength or band of wavelengths and a stable response from the radiation detector. It is well known to one skilled in the art that physical and chemical changes in the source construction over time cause changes in source radiation intensities. Contamination deposits on the surface of optical components also attenuate radiation intensities, and physical movement of radiation sources and/or optical elements change the radiation flux striking measuring detectors. The measurement process is further biased by signal response produced by radiation detectors sensitivities due to aging or from temperature changes, and variations in signal gain from the signal conditioning electronics are common.
Photometer response measurements can be described in terms of radiation production, non-sample material related radiation loss, sample material related radiation loss, and detector response factors as follows: EQU R.sup.Op =I.sup.Op *F.sup.Op *L.sup.Op *D.sup.Op *G.sup.Op *X(n).sup.Op(2)
where I represents the source radiation intensity at a specified measuring wavelength or band of wavelengths; F is the fraction of radiation directed from the radiation source to the radiation detector; L is the fraction of light directed to the detector not absorbed by contamination materials; D is the sensitivity of the radiation detector to the measuring wavelength (detectivity); G is the detector signal conditioning gain; and X is the fraction of radiation not absorbed by the sample material.
When no sample material quantity is present in the measuring instrument, a standard reference response is produced as follows: EQU R=I.sub.0.sup.St *F.sup.St *L.sup.St *D.sup.St *G.sup.St *X(n.sub.0).sup.St(3)
An instrument response ratio of material quantity measurements to the standard reference of equation (3) gives a material response function, f(mp), as follows: ##EQU2##
Factors which are eliminated by cancellation from equation (4) are said to be "common mode". Elimination or minimization of the measurement parameters present in equation (4) that are not dependent on material sample quantities reduces measurement errors and in turn increases the stability of quantitative photometric instruments. As will be shown in the following discussion of the related art, a variety of photometers have been proposed which manipulate the measurement and reference parameters in attempts to design reliable, stable photometers.
Some common instrumentation parameters which change independently of sample material quantities are source intensity, temperature of measuring volumes, temperature of filter devices, temperature of radiation detectors, and pressures of measuring volumes. Attempts have been made in the art to stabilize instrumentation parameters such as the device disclosed in U.S. Pat. No. 4,233,513 to Elder. However, frequent calibration is necessary to overcome changes in the instrument due to physical and chemical changes. Various calibration methods and techniques have been set forth in the art in attempts to overcome such problems.
In general, calibration procedures require interruptions in material monitoring time. Calibration procedures add significant costs to the product design and the operation of the measuring system. In some applications, interruptions of material monitoring can generate unacceptable risk in critical applications such as gas monitors used during medical surgery. Eliminating periodic calibration, or extending the calibration interval will reduce both the time required for measurements and the operating costs.
Other attempts to improve photometric devices have been made by incorporating compensation techniques to the methods and apparatuses. Compensation techniques add measurement variables to photometers which serve to stabilize or correct for measurement parameters which are not intended to change after instrument calibration. For instance, U.S. Pat. No. 4,355,234 to Fertig et al. and U.S. Pat. No. 4,598,201 to Fertig et al. include dual radiation beam configurations which attempt to eliminate instrument drift by adding a reference path to the photometer. Dual beam photometer designs are effective in removing common mode errors due to sample matrix effects, detector response changes and response signal gain changes. The increased design complexity of dual beam instruments generally add significant cost to a photometer design. Dual beam photometers can also increase measurement error due to relative changes in the two beams which arise due to unequal absorption of measuring radiation, contamination of non-optical elements, and mechanical movement of optical elements.
Pressure variations have been employed to create stable signals as described in U.S. Pat. No. 4,500,207 to Maiden and U.S. Pat. No. 4,975,582 to Mount et al. These techniques rely on the long term stability of pressure regulators or pressure modulators to obtain reliable results. However, mechanical components used to generate pressure variations produce undesirable noise, add significant cost, and fail through long term mechanical wear.
Dual wavelength photometers are yet another alternative means to compensate for measuring parameters that are independent from the measured sample material. When adding additional wavelengths to measuring instruments, additional signal responses are obtained. U.S. Pat. No. 5,341,214 to Wong describes a dual wavelength photometer which utilizes a single detector, single beam, and dual wavelengths to analyze gases. However, requirements that intensity ratios of measuring radiations and reference radiations remain constant is a serious limitation to conventional dual wavelength, single beam, single source configurations. Over extended operating periods, radiation source temperatures will change thereby creating changes in measuring and reference radiation intensity ratios. Frequent calibration is therefore required.
In U.S. Pat. No. 3,745,349 to Liston and U.S. Pat. No. 3,895,233 to Boll et al., measuring instruments are disclosed that utilize two radiation sources in order to provide measuring and reference radiation beams with no moving parts. The presence of radiation beams alternate between measuring radiation and reference radiation, which cause measuring signal responses and reference signal responses to be alternately produced. The two radiation sources establish measuring radiation and reference radiation originating from physically distant sources. Relative movement of the sources change light transmission ratios from the measuring radiation source and reference radiation source to the radiation detector. Detectors, and other elements of information channels, are subject to short-term drifts and instabilities that make them respond differently, at different times and in unpredictable ways. When one or both channels exhibit response changes or response components that have no counterpart in the response of the other channel, there is no common mode cancellation of their extrinsic effects on the measurement. Frequent calibration is therefore needed for reliable measurements.
U.S. Pat. No. 4,648,396 to Raemer and U.S. Pat. No. 5,153,436 to Apperson et al. disclose instrumentation with single beams, dual wavelengths, and two detectors. Source radiation containing measuring radiation and reference radiation is directed simultaneously through a sample measuring volume. The measuring radiation and reference radiation are then substantially separated by optical components and directed respectively to measuring radiation detectors and reference radiation detectors. There are no common mode variables in the Raemer and Apperson designs. Apperson et al describe temperature controlled detectors to enhance stability. Raemer relies on the presence or absence of material sample to circumvent the need for frequent calibration. Not all material quantification applications can conveniently remove the sample material at periodic intervals.
U.S. Pat. No. 4,057,734 to Barringer and U.S. Pat. No. 5,381,010 to Gorden disclose dual wavelength, dual beam, dual detector photometers for detecting gases. The monitoring radiation and measuring radiation do not traverse a common optical path but travel in two separate information channels. Therefore, changes in component positions or physical properties that have no counterpart in the response of the other channel can effect one or both information channels to exhibit response changes.
U.S. Pat. No. 4,300,049 to Sturm discloses single source, single path, dual detector, three wavelength instruments for analyzing paper sheet. Although some common mode parameters may be eliminated, the material response function utilized is dependent on the relative spectral sensitivity of the radiation detectors over extended periods of time. Changes in spectral sensitivity will introduce measurement error. Radiation loss due to contamination deposits on optical surfaces will also introduce measurement error in the Sturm photometer. The construction of the instrument is complex, requiring three radiation sources to achieve measuring stability.
The present invention contemplates new and improved methods and apparatuses for obtaining stable and reliable material sample measurements which are simple in design, effective in use, and overcome the foregoing difficulties and others while providing better and more advantageous results.